when I was little, my grandma made me draw circles with markers every day. Now I'm very good at it!
What can I contribute that somebody hasn't already said... Ah yes, draw them quickly. Visualize how big you want it, then complete the circle with one fluid stroke, or more, but without taking the pencil off the surface.
Another mental technique I use (usually for large circle) is to just focus on swinging my arm in a circular motion, rather than focusing on what's on the paper.